Photovoltaic devices function by converting radiation absorbed from sunlight into electrical power through photon absorption by semiconductor materials that exhibit the photovoltaic effect. Solar cells include photovoltaic devices that convert sunlight (i.e., photons originating from the sun) into electricity. The growing demand for renewable energy resources continues to drive development of cost-effective and high-efficiency photovoltaic cells for use in solar cells and arrays of photovoltaic devices.
Solar cells can be broadly classified into types that include silicon solar cells, thin-film solar cells and compound solar cells. A so-called “thin-film solar cell” (TFSC), also referred to as a “thin-film photovoltaic cell” (TFPV), is a solar cell that is made by depositing one or more thin layers (i.e., thin films) of photovoltaic material on a substrate. The photovoltaic materials used in TFSCs may be produced in a variety of crystalline and non-crystalline forms. Although the crystalline materials have exhibited high conversion efficiencies, the cost of production may prohibit widespread use in TFSCs.
Several crystalline materials have been increasingly studied for use in fabrication of the TFSCs due to their potential for stability, reliability and performance. For example, chalcopyrites (e.g., CuInS2, CuGaS2, and CuInSe2) have band gaps that correlate well to the solar spectrum, have large absorption coefficients and good photostability and, thus, have great potential for use in the TFSCs.
To improve efficiency of the TFSCs, it is important to form the chalcopyrites having desired material properties. Various techniques are known in the art for depositing the chalcopyrites on substrates, such as co-evaporation, sputtering, sulfurization, ion plating and chemical processes. It has been shown that crystal structure and size may affect the optoelectronic properties of chalcopyrites and, thus, the performance of the TFSCs including such materials. Furthermore, intrinsic defects in the chalcopyrites may affect electrical, optical and structural properties.
Methods for forming photovoltaic devices using chalcopyrite nanoparticles have been disclosed. The photovoltaic devices formed by such methods include one or more layers of a photovoltaic material formed from the chalcopyrite nanoparticles. To obtain a photovoltaic material having a desired particle (i.e., grain) size, a two-part annealing process is performed—the first part promoting adhesion of the chalcopyrite nanoparticles and the second part converting the chalcopyrite nanoparticles to a unitary chalcopyrite structure. The ability to form a chalcopyrite material having a dense structure and a large grain size would represent a significant improvement in the development of photovoltaic devices.